In many applications, it is desirable to convert fixed color images, for example photographic negatives or prints, into electrically storable images. One method for accomplishing this is to use a monochrome image sensor and three color filters, which are placed into the light path one at a time, to obtain red, green, and blue (RGB) color separation images which can be combined to form a single color image. A typical system works by illuminating the fixed image and focusing the image through a lens and an RGB color filter wheel onto an image sensor. One example of such a system is described in U.S. Pat. No. 2,478,598 entitled, "Variable Color Filter Assembly for Television Transmitting Systems" by Frank J. Somers. It is desirable in such systems that the video signals representing the red, green and blue component colors produced by the system be equal, for white or grey portions of the color image. However, the light transmitting efficiency of the filters and the spectral response of the image sensor are typically different for red, green, and blue. U.S. Pat. No. 2,478,598 teaches a method for compensating for these differences by changing the relative sizes of the rotating red, green, and blue filters, and thus the relative sensor exposure times for the red, green, and blue separations. This provides more uniform signal levels for the red, green, and blue separations from neutral image objects, otherwise referred to as providing better "white balance". Because the shutters for the red, green, and blue filters described in that patent are affixed to the rotating filter wheel which spins at a constant velocity, only a limited range of exposure times can be obtained.
A second patent of interest for its teachings is U.S. Pat. No. 4,713,683, entitled "Illuminating and Synchronizing Device for Color Imaging Equipment" by Fujimori et al. The device of that patent utilizes a color filter wheel and an illuminating light source having an intensity level which varies when the different color filters are positioned in front of the image sensor. The color filters have different sizes, chosen according to the sensitivity of the image sensor to the particular color of the filter.
When converting color negative film images into electrical signals using a charge coupled device (CCD) image sensor with a full frame architecture, the differences between the red, green, and blue exposure times required to obtain proper white balance are especially significant. This is because normal color negative films have a yellow mask which absorbs most of the blue light, and because full frame image sensors have a significantly reduced response to blue light, relative to red and green light, due to the spectrally selective absorption of the polysilicon layer which overlays the sensor's photosites. Furthermore, the light sources typically used in such systems contain tungsten bulbs, which provide very little energy in the blue spectral region, compared to the energy they emit in the red and green regions. Fluorescent light sources provide significantly more blue energy, but unlike tungsten lamps, the light output of fluorescent bulbs fluctuates significantly in response to the phase of the AC line voltage. The aforementioned causes undesirable variations in the illumination level of the sensor. What is needed is a way to provide a light source with a high content of blue spectral energy, and a means to provide the needed range of exposure times for red, green, and blue to obtain proper white balance, while also providing consistent sensor exposures which do not change from one image capture to the next.